This invention relates to a decanter centrifuge. More specifically, this invention relates to a decanter centrifuge with structure for reducing the moisture content of a discharged cake or increasing solids fraction, while maintaining a relatively high cake throughput rate. This invention also relates to an associated method for operating a decanter centrifuge.
A decanter centrifuge generally includes an outer bowl, an inner hub carrying a worm conveyor, a feed arrangement for slurry to be processed, and discharge ports for cake solids and clarified liquid. The bowl includes a cylindrical section and a conical beach section. The bowl and the hub are rotated at high, yet slightly different angular speeds so that heavier solid particles of a slurry introduced into the bowl are forced by centrifugation into a layer along the inner surface thereof. By differential rotation of the worm conveyor and the bowl, the sediment is conveyed or scrolled to a cake discharge opening at the smaller, conical end of the bowl. Additional discharge openings are provided in the bowl, usually at an end opposite of the conical section for discharging a liquid phase separated from the solid particles in the centrifuge apparatus.
One of the goals in centrifuge operation is to produce cakes with a low moisture content. One proposed method, published in Research Disclosure, March 1993, Number 347, for reducing cake moisture content entails the disposition of a flow control structure proximate to the cake discharge port to reduce the volume flow rate of the cake by 25% to 75%. The flow control structure could be a ring shaped dam extending radially outwardly from the axis of the bowl, a dam disposed between two turns or wraps of the conveyor, an increased beach climb angle, an increased conveyor blade thickness, or an increased or decreased conveyor helix angle. It was asserted that by decreasing the volume flow rate of the solids by about one-half, or between 25% and 75%, the velocity at the interface between the liquids and the sedimented solids is in the reverse direction, i.e., towards the pool and away from the cake discharge port. Liquid from the pool and liquid expressed from the cake layer are drained back into the pool rather than carried out of the bowl with the sedimented solids.
Although a drier cake is obtainable by the published technique discussed above, the problem generated by such a cake flow control solution is that the cake production rate or throughput is reduced, thus increasing costs and reducing efficiency.
It is also known to form a dip weir along the outer surface of the conveyor hub, at or about the location of the junction between the cylindrical and conical sections of the bowl, to serve in selecting the driest portion of the cake at the discharge end of the bowl. The dip weir blocks the transport of the sludge cake in such a manner that the most compacted part of the cake passes under the dip weir and reaches the cake discharge opening. The dip weir also acts to provide the appropriate resistance to cake flow so as to maintain a large cake thickness upstream of the weir, creating high compacting pressure and long residence time. In conventional practice, the dip weir is fixed to the hub so that the radial gap between the outer edge of the dip weir and the inner surface of the bowl is constant or fixed. The designer must position and dimension the weir to minimize cake moisture content while not excessively increasing cake transport resistance through the gap so as to unduly limit the solids capacity of the machine. The optimal gap height depends on the nature of the cake, the G level, and the cake flow rate or solids throughput. The designer is forced to guess at the correct gap height, guided somewhat by past experience.